The purpose of the study is to determine whether heat and x-ray can offer a therapeutic advantage over x-irradiation alone in a murine tumor model and to determine what conditions will optimize that advantage. Two transplantable C3H mouse mammary tumor lines (one slowly proliferating with a large hypoxic cell fraction and the other rapidly growing with a very small hypoxic fraction) are implanted and grown on the flank and then treated with x-ray followed by 44 degrees C for 15 minutes. Water bath heating with a special technique that optimizes minimum tumor temperature to within 0.1 degrees C of the bath temperature is used. TCD50/120 is determined with and without heat and the thermal enhancement ratio calculated. Tumor results are compared with thermal enhancement ratios for small intestinal epithelium based on the crypt cell survival assay at 3 1/4 days post-treatment. An in situ heating method for small bowel witha time-temperature curve and temperature uniformity directly comparable to the tumor heating technique has been developed which avoids exteriorization of the treated bowel and permits fractionated treatment. A similar assay of the thermal enhancement ratio for damage to mouse spinal cord is intended, but the basic assay for spinal cord injury has not yet proven sufficiently reliable. An assay based on rat spinal cord appears more feasible and may be used and other assays of damage to slowly proliferating normal tissues in the mouse are being evaluated. The therapeutic gain derived from the use of hyperthermia in these systems will be calculated and modification of sequence, interval and treatment fractionation will be investigated to determine how heat and x-ray can best be combined to minimize relative normal tissue injury.